Interface circuitry for nanowire based sensors is typically important for reading out conductance changes that occur in the nanowires when detecting the density or reaction between macromolecules such as Deoxyribonucleic acid (DNA) or protein. Typically, the design of interface circuits with low noise, large dynamic range and high sensitivity to small changes in the conductance of nanowires is practically challenging.
A typical nanowire sensor behaves electrically as a resistor. Its resistance value comprises of three components ie. a baseline resistance RBL, which can vary from a few MΩ to a few GΩ depending on fabrication conditions such as the doping density, a deviation ΔRBL from the baseline resistance RBL, which is caused by temperature variation, and a resistance variation ΔR due to bonding of charged macromolecules to nanowire receptors. Hence, the nanowire sensor impedance can be written as Rnw=RBL+ΔRBL+ΔR. The resistance variation ΔR is measured when the nanowire sensor is exposed to a solution containing a specific biomolecule. The resistance Rnw can typically increase or decrease by as small as about 1% of the baseline value RBL depending on the net charge of the macromolecule (ie. the biomolecule) and the semiconductor type (ie. p- or n-type) of the sensor. The typical large variation of the baseline impedance RBL, combined with the typical small variation ΔR to be measured, thus necessitates a high precision detection requirement for an electrical read-out interface circuit, in order to provide an adequate margin for a subsequent data processing block, such as a microcontroller, to recognize the macromolecule contained in the solution and the concentration of the marcomolecule.
M. Grassi, P. Malcovati, and A. Baschirotto in “A 160 dB equivalent dynamic range auto-scaling interface for resistive gas sensors arrays,” IEEE Journal of Solid-State Circuits, vol. 42, no. 3, pp. 518-528, March 2007, describe an integrated wide dynamic-range interface circuit for gas sensors that achieves an accuracy of about 0.1% over a sensor resistance range of more than 5 decades. However, the accuracy is at a cost of using a practically sophisticated calibration system, an increased die area and increased realization costs.
A. Flammini, D. Marioli, and A. Taroni in “A low-cost interface to high value resistive sensors varying over a wide range,” IEEE Transactions on Instrumentation Measurement, vol. 53, no. 4, pp. 1052-1056, August 2004, describe a low-cost interface which is implemented by using a similar approach to M. Grassi et al for high-value resistive sensors. However, the work of A. Flammini et al is not an integrated circuit and is undesirably sensitive to parasitic capacitances. Thus, there is a problem of a large parasitic capacitance affecting sensing accuracy.
Hence, there exists a need for an interrogation circuit for a nanowire sensor array and a method for interrogating a nanowire sensor array which seek to address at least one of the above problems.